The goal of the research proposed in this request for renewal of an ADAMHA RSDA is to understand how gonadal steroids act on the brain to modify the social behaviors of mammals. It is pursued by focusing on how testosterone (T) activates social behaviors in adult males and effects their sexual differentiation. Parallel analyses compare T's effects on a sexually dimorphic area (SDA) in the gerbil hypothalamus with its effects on masculine sexual behavior and scent marking, a form of olfactory communication. The SDA is a set of hormone-accumulating cell groups that responds to T morphologically an histochemically in adulthood. At least one subdivision of the SDA, the SDA pars compacta (SDApc), also sexually differentiates under the control of T during early development. While structural sex differences have been identified in the hypothalamus of at least nine species, including humans, evidence linking the SDA to hormonal control of masculine social behavior is stronger than in any other species. Thus it is the best model available for attempting to identify cellular markers of T action and neural pathways involved in these behaviors. During the period of requested support, this model will be developed further by (1) determining if cell-body lesions of the SD disrupt mating and marking as electrolytic lesions do, and if the deficits are related to damage to specific subdivisions; (2) determining if it is easier to elicit marking or mating with hormone implants in the SDA than in adjacent areas, and if the effects vary for different subdivisions; (3) studying the distribution of muscarinic receptors in the SDA and their relationship to the facilitation of scent marking by muscarinic antagonists; (4) determining which afferents and efferents are important for mating or marking and if any mediate the effects of T on the SDA; (5) determining if SDA cells directly link the pertinent afferents and efferents; (6) determining if the pertinent input, output or intervening SDA cells accumulate T or its metabolites; (7) attempting to identify the transmitters/modulators of the behaviorally important pathways; (8) studying the effects of pudendal nerve cuts on the SDApc; and (9) continuing research on sexual differentiation of the SDA and SDApc. Continued development of the candidate in the area of chemical neuroanatomy will be achieved through formal coursework, consultation with other scientists, and by bringing new techniques into practice in the lab as they aid in exploring questions posed by behavioral studies.